SONAR AND RADAR 



pulse has returned to the ship's hull a further echo re- 

 turns somewhat later. On first seeing such a record, an 

 experienced physicist might surmise that the pulse had 

 made two round trips through the depth of water under 

 the ship's hull— down to the bottom, up to the surface, 

 down to the bottom again, and finally back as a second 

 echo. This can indeed happen, but then the time of ar- 

 rival of the second echo is almost exactly twice that re- 

 quired by the first. Many of the false bottoms that seem 

 to lie below the real bottom result from echoes return- 

 ing at other times than twice the travel time of the first, 

 direct echo. What really happens under certain condi- 

 tions is that some of the sound energy penetrates into 

 the mud or sand of the ocean floor, travels downward 

 through it, and is then reflected back again by some 

 sudden discontinuity such as a layer of rock of different 

 hardness or density. Making due allowances for the 

 velocity of sound transmission through the material 

 just below the bottom of the ocean, geologists can esti- 

 mate rather accurately the depth below the bottom at 

 which this discontinuity occurs. Without even intending 

 to do so, designers and users of echo sounders have thus 

 hit upon a method of echolocation underground. 



Quite purposefully and for many years, other geolo- 

 gists have been studying the transmission of sound waves 

 through miles of the earth's crust. Earthquakes produce 

 vibrations that can be detected by deUcate vibration de- 

 tectors known as seismographs. So do man-made ex- 

 plosions if they are sufficiently violent. Blasting in mines 

 and quarries can be detected miles away, and the 

 seismographic detection of nuclear explosions has now 

 become a matter of major importance, a hotly debated 

 issue at international conferences. By comparing the 

 vibration records resulting from earthquakes at different 

 points around the world, it is possible to deduce that 



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